Process for electrolytically coloring aluminum and aluminum alloys

Abstract

Aluminum or aluminum alloy is first anodized to form an oxide film thereon. With the pores in this oxide film unsealed, the basis metal is then electrolyzed by use of alternating current at a preselected starting voltage, and this voltage is lowered at least once before completion of the electrolytic coloring of the basis metal. The electrolytic solution in which the electrolysis is effected contains either at least three metallic salts or at least two metallic salts and a strong reducing compound.

What is claimed is: 
     
       1. A process for electrolytically coloring aluminum or aluminum alloy which comprises anodizing a desired basis metal, and electrolyzing the anodized basis metal in an acidic aqueous electrolyte by use of a constantly applied alternating current, the voltage at which the anodized basis metal is electrolyzed being lowered at least once by a value of from about 1 to 10 volts but not to a sustained zero potential in the course of the electrolysis. 
     
     
       2. The process as recited in claim 1, wherein the anodized basis metal is electrolyzed at a starting voltage in the range of from about 6 to 50 volts. 
     
     
       3. The process as recited in claim 1, wherein the voltage is lowered within about 2 minutes following the start of the electrolysis. 
     
     
       4. The process as recited in claim 1, wherein the electrolyte contains at least three metallic salts in a concentration of 5 to 500 grams per liter. 
     
     
       5. The process as recited in claim 4, wherein the metallic salts are selected from the group consisting of inorganic and organic acid salts of nickel, cobalt, chromium, copper, magnesium, iron, cadmium, titanium, manganese, molybdenum, calcium, vanadium, tin, lead, and zinc. 
     
     
       6. The process as recited in claim 1, wherein the electrolyte contains at least two metallic salts and a strong reducing compound selected from the group consisting of dithionites, thiosulfates, bisulfites, sulfurous acid, sulfites, thioglycolic acid, and thioglycolates in a concentration of from about 0.05 to 10 grams per liter. 
     
     
       7. The process as recited in claim 6, wherein the metallic salts are selected from the group consisting of inorganic and organic acid salts of nickel, cobalt, chromium, copper, magnesium, iron, cadmium, titanium, manganese, molybdenum, calcium, vanadium, tin, lead, and zinc.